Temperature control in a diode laser is important for several reasons. First, temperature control is important because a change in temperature can strongly and adversely effect a laser's power output. For example, a diode laser's power output can be increased by increasing the bias current across the diode laser, but this approach is limited because the increase in current will lead to an increase in the diode laser's temperature. Typically, at least half of the electrical power applied to bias a laser will be lost as heat within the diode. This in turn leads to an increase in the threshold current necessary for sustaining laser oscillation. As a result, the relationship between the bias current and output power becomes nonlinear as the bias current is increased, and ultimately will saturate at a high bias-current level. This is the so-called "rollover" effect; as the bias current is increased, the temperature in the active region increases, and so the power output begins to drop off.
Second, temperature control is important because temperature is related to the output spectrum. Overheating the active region can lead to both a shift of and a broadening of the emitted spectrum. Thus, the ability to control the output spectrum, and consequently, the ability to use the resulting output spectrum, decreases as the active region increases in temperature.
To reduce temperature-related problems in a diode laser, known thermal stabilization systems have been designed to operate with the diode lasers. These thermal stabilization systems include a thermal sensor adjacent to the laser heat sink to provide a feedback signal, and a thermo-electric cooler also adjacent to the laser heat sink where the sensor is fixed. Such thermal stabilization systems are able to support the relatively stable active region temperature under conditions of variable ambient temperature.
Said another way, known thermal stabilization systems have thermal sensors adjacent to the heat sink. These thermal sensors help stabilize the temperature of the heat sink at the spot where the sensor is mounted. In these known thermal stabilization systems, thermal sensors are placed at the heat sink because this is the simplest geometry, and it was generally believed that this was the only possible way to use the thermal sensors. Additionally, those skilled in the known art did not think that the temperature gradient between the thermal sensor and the laser body was substantial enough to degrade performance. For example, it is generally known in the art to use sensors that are not removable. The sensors are fixed in the copper part of the laser fixture adjacent to a laser heat sink. It was generally believed that this was the only possible way to measure the temperature in the laser.